Rolling bearing manufacturers decide upon the type of heat treatment to be used for rolling bearing components depending on the particular application for the rolling bearing, or the types and sizes of the roller bearings. For through hardened bearing steel components, two heat-treating methods are available--martensite hardening or austempering. Component properties such as hardness, microstructure, retained austenite content, and dimensional stability are associated with or affected by the particular type of heat treatment employed.
The following table provides an overview and comparison of various component properties associated with different heat treatment methods.
Retained Dimensional Hardness Austenite Stability Method HRC % .DELTA. D/D Martensite 62 . . . 55 8 . . . 16 +60 .mu.m/100 .mu.m Hardening (tempered normally) Martensite 58 . . . 62 .ltoreq.3 +15 .mu.m/100 .mu.m Hardening (stabilized) Austempering 58 . . . 63 .ltoreq.3 +15 .mu.m/100 .mu.m
Austempering and martensite hardening (stabilized) do not differ significantly with respect to hardness, retained austenite and dimensional stability. Austempering, however, has better toughness properties than martensite hardening and also has a different residual stress state. Both methods also suffer from various disadvantages.
At present, both methods are always embodied completely. That is, either a martensite transformation or a bainite transformation takes place. In addition, time-temperature combinations, quenching and transformation in the bainite stage are done in the manner specified in time-temperature transformation diagrams such as FIG. 1 from the Atlas zur Warmebehandlung der Stahle, the Atlas of Heat-Treatment of Steels.
The time required to attain the desired component properties is relatively long. For through hardened bearing steel components, the time required can be more than four hours, both for austempering and in martensitically hardened stabilized components.
The previously known methods described above preclude one another and so it is not possible to combine the properties of austempering and martensite hardening. Nor is it possible with current methods to reduce the total process time.
A need thus exists for a method of heat-treating a steel or cast iron component, including a through hardened bearing steel component, in a shorter time yet with the component possessing the desired properties.